INTEGUMENT OF VERTEBRATA. 417 



towards the anterior, and that between tlie thigh and leg towards 

 the posterior, 



In addition to these general modifications of the appendages 

 there are other changes, which arc confined to smaller divisions, and 

 are explicable by special variations in physiological activity. When 

 the hind-limbs are greatly developed, they perform the more com- 

 plicated function of a springing organ, as in frogs ; or they may be 

 converted into the chief organs of support for the body, in such a 

 way that the fore-limbs, so far at least as terrestrial locomotion is 

 concerned, may get to play a subordinate part, or even lose this 

 function altogether. This arrangement obtains in Birds, where it 

 has been attained to through a large number of intermediate steps, 

 which have been made out in fossil Reptiles ; the fore-limbs in the 

 Carinatae have taken on the function of a flying organ. 



Integument, 

 § 319. 



In the primitive stage iu the Vertebrata the investment of 

 the body has the character of a cellular layer, the external 

 germinal layer — the ectoderm. At a further stage in develop- 

 ment this cellular layer is connected with a layer of connective 

 tissue derived from the mesoderm, and the two together form the in- 

 tegument of the Vertebrata, and take an equal share in the formation 

 and further development of various organs. 



There are two layers in this integument (cutis), as might be 

 inferred from its mode of origin : a superficial epidermis which is 

 homologous with the epithelial structures iu the Invertebrata, and 

 which is directly derived from the ectoderm, and a deeper-lying* 

 layer of connective tissue, the corium, the deepest, and looser, 

 layer of which forms the sub-integumentary tissue. The corium is 

 strengthened by the formation of plexuses between its fibrous bauds. 

 The blood-vessels, and nerves of the skin, together with various 

 sensory organs, are scattered in it, as are also glandular organs. 



The corium is frequently pigmented. It varies greatly in 

 thickness, and in microscopic structure. One of the more notable 

 variations is a lamellar striation seen in Fishes, Amphibia, and 

 Reptiles, where perpendicular fibrous bands divide the layers into 

 partitions. Among the special structures are the wart-like elevations 

 seen on its surface, which vary from low hillocks to long conical 

 processes. These dermal papillae give rise, in different divisions 

 of the Vertebrata, to a number of variously complicated organs. 



Contractile form-elements (smooth muscular fibres) are also 

 found in the corium of Birds and Mammals. Another modification, 

 which obtains in the cutis, is due to a change in texture ; pai'ts of 

 it are ossified into hard structures, and bony plates are developed in 



2 E 



